The present invention relates to a ceramic glow plug to be fitted to a diesel engine mainly for the purpose of starting it, and more particularly, to a ceramic heater as a heating element of the glow plug.
In a diesel engine, to increase its starting performance, a glow plug is provided to a precombustion or swirl chamber, for example, and is made red-hot by passing electricity therethrough whereby when a fuel is injected into the chamber, part of the fuel is burned on coming into contact with the hot glow plug, thereby preheating the whole fuel. It is therefore required for the glow plug to have rapid heat-up characteristics at the time of starting. Furthermore, since there is a tendency that even after the starting the glow plug is used as an after glow for stabilization of combustion for long periods of time, it has been increasingly needed to increase the durability of the glow plug.
As a rapid heat-up type glow plug meeting the above requirements, a ceramic glow plug has heretofore been known in which a ceramic heater produced by embedding a heating wire made of a high melting point metal, tungsten (W), in powdered ceramic followed by sintering is used as a heating element. The structure of the ceramic glow plug is generally as shown in FIG. 1.
That is, a ceramic glow plug 1 comprises a ceramic heater 3 comprising a ceramic sintered body with a heating wire coil 2 embedded therein, a metallic external cylinder 4 in which the ceramic heater 3 is fitted and retained and to which one end of the heating wire coil 2 is electrically connected, a fitting metal 5 to which the metallic external cylinder 4 is brazed, thereby constituting a negative (-) electrode, and a center rod 6 electrically insulated from the fitting metal 5, to which the other end of the heating wire coil 2 is connected, thereby constituting a positive (+) electrode.
In conventional glow plugs having the structure as described above, the ceramic heater 3 is designed so that a heater part 3' in which the heating wire coil 2 is embedded and which becomes red-hot on passing electricity through the heating wire coil 2 extends beyond the edge of the metallic external cylinder 4. The projection length l is usually from 12 to 15 mm in the case where the battery voltage of a power source is 12 V. Since, however, the heater part 3' is fitted so that it is exposed in a combustion chamber, such as a swirl chamber, if the glow plug is designed with heater projection part 3' having a large outside diameter D to provide after glow for long periods of time, the exposed projecting portion prevents the formation of swirl in the chamber, thereby exerting adverse influences on combustion of the diesel fuel. This further gives rise to problems such as a reduction in engine output, a decrease in specific fuel consumption, and an increase in the amount of harmful exhaust gases. For this reason, it is preferred for the heater part 3' to be designed so that its external diameter D is as small as possible and the projection length l from the metallic external cylinder 4 is as short as possible.
When, however, the heater part 3 is reduced in its external diameter D, it is easily broken by the severe heat cycling, rapid heating and rapid cooling, in the chamber and vibration, impact and so forth to which it is exposed during driving. That is, a mechanical strength problem as described above is encountered in decreasing the external diameter D of the heating part 3'. In order to overcome the above problem, a glow plug as shown in FIG. 2 (in which the same parts as in FIG. 1 are indicated by the same symbols) has been proposed in which the metallic external cylinder 4 holding the heater part 3' is increased in length, thereby decreasing the projection length l.sub.1 and reinforcing the heater part 3'. In this glow plug, however, a portion of the heating wire coil 2, corresponding to the length l.sub.2 as shown in FIG. 2 is covered with the metallic external cylinder 4 and, therefore, a heat-generation efficiency is reduced. Moreover, the temperature of the above-defined portion of the heating wire coil 2 rises excessively, causing a problem that a brazing material used in connecting the metallic external cylinder is melted. If the projection length of the heater part is merely shortened without changing the material and wire diameter of the heating wire, the heating wire coil embedded in the heating part is inevitably shortened. This leads to a reduction in resistance value and causes a problem that the necessary amount of heat cannot be generated.